The present invention relates to a method of measuring thin film thickness, and more particularly to a reflective microscopy film thickness measurement method.
Thin films are broadly applied in the industry, for example, the flat panel display (FPD) industry. The color filter includes a plurality of films, such as R, G, B color filter, photo spacer (PS), multi-domain vertical alignment (MVA) material, and indium tin oxide (ITO), each having a different function. The color filter film determines the color performance, the thickness of the PS film influences the operation performance of liquid crystals, and the thickness of the MVA film further influences the viewing angle performance of the liquid crystal panel. Therefore, it has become a critical technique to accurately detect the thickness and uniformity of each film.
The thin film thickness measurement technique has been developed for several years and has been widely applied. In recent years, the area of the sample under measurement becomes increasingly larger, and the process speed is increasingly increased, and thus a quick and accurate measurement becomes more and more important. The conventional thin film thickness measurement techniques mostly adopt a single point measuring method. When the film thickness uniformity of the entire sample is measured, the probe or sample must be moved in two dimensions, which cost too much time in measuring. Further, along with the development of the process technique, the film thickness measurement and monitoring in small region (e.g., pixel size) is becoming more and more important. Therefore, a measuring method capable of quickly and accurately detecting the film thickness uniformity of a sample with a larger area and also capable of monitoring the film thickness in small region is urgently needed in the industry field.
Among the film thickness measurement techniques, the spectral reflectance measurement method is performed by measuring a reflectance spectrum when an incident direction of a light source is perpendicular to a sample. When a broadband light source is incident in perpendicular to the sample, the reflected light from the bottom of the film is refracted to the air, and interferes with the reflected light from the surface of the film. As shown in a schematic view of a reflectance spectrum in FIG. 1, a horizontal axis in the figure represents the wavelength in a unit of nm, and a vertical axis represents the reflectance. When the light reflected from the bottom surface of the film and the light reflected from the surface of the film are in phase, a constructive interference is generated, and the reflectance has an extremely large value, i.e., crest 10 in FIG. 1. However, when the two are out of phase, a destructive interference is generated, and the reflectance has an extremely small value, for example, trough 12 in FIG. 1.
The interference is relevant to optical path difference (OPD), the OPD is relevant to film thickness and reflectance of the film property, and the reflectance is a function of the wavelength. Thus, under the same film property conditions, along with the increase of the film thickness, the number of crests of the reflectance curve is increased, which can be seen from FIGS. 2A, 2B, and 2C. FIG. 2A is a reflectance spectrum obtained at a thickness of 500 Å (10-10 meter), FIG. 2B is a reflectance spectrum obtained at a thickness of 5000 Å, and FIG. 2C is a reflectance spectrum obtained at a thickness of 20,000 Å. It can be seen clearly from comparison that the thicker the thickness is, the more the number of crests and troughs in the same wavelength interval is. Therefore, the thickness of the measured film can be obtained by performing curve fitting on the reflectance spectrum of the known thickness and the measured reflectance spectrum.
The above reflective measurement method of thin film thickness can be used to obtain the thickness of the thin film. However, in actual measurement, the read reflectance spectrum may have poor spectrum resolution and spatial resolution due to the optical aberration. That is to say, when a single-wavelength light source after passing through a measurement system is irradiated to a thin film and reflected to a photo-detecting element, a spectrum read by the photo-detecting element may be the emitted single-wavelength reflected light theoretically. But in practice, after the single-wavelength light source passes the optical path of the entire measurement system, a reflectance interference spectrum much larger than the single-wavelength bandwidth is read on the photo-detecting element due to the optical aberration. Referring to FIG. 3, it can be seen that the wavelength range in the reflectance interference spectrum is spread to about 4 nm, which varies depending on different measurement systems.
Therefore, when the thickness of a thin film is measured by a light source of a broadband spectrum passing through a measurement system, a spread phenomenon is generated in the reflectance interference spectrum read by the photo-detecting element, as shown in FIG. 4. The spread phenomenon is formed by the accumulation of single-wavelength light sources, and thus the measured spectral resolution may be deteriorated due to the optical aberration. Besides, the spatial image resolution has the same situations.